insert_mesh

[mdsumner]

• ensure index is within range for coordinates (0-based, warn if incomplete set but in range)
• wrap insert_mesh, add internal _cpp function
• wrap converters for silicate types
• work out returning segments and vertices (we can't iterate faces for the info->index as done tri_xy(), because this mesh gets new vertices inserted - though might be worth turning of the conforming application, and do that first)